As it eventually worked out, the inter-universal messenger had to be constructed from the sub-microscopic level on up out of fundamental nuclear particles which came as close to being nothing at all as either universe would ever be likely to provide: zero-spin particles with various charges and masses, and neutrino/anti-neutrino pairs. Even detecting that the object was present at all after it had been built was an almost impossible task, for neutrinos and anti-neutrinos have no mass and no charge, consisting instead partly of spin, partly of energy of translation; it did no good to try to visualize such particles since like all the fundamental particles they were entirely outside of experience in the macroscopic world. Matter was so completely transparent to them that stopping an average neutrino in flight would require a lead barrier fifty light years thick.